The simple answer is by winning
more seats
in the House of Commons than all the other parties put together. At the moment there are 650 seats up for grabs, which means 326 seats are needed to win an overall majority.

Also called a "seat", in a general election, this is where the political battles are fought. Voters in each constituency choose just one MP. The
constituencies
are towns or areas all of roughly the same size.

In the 2005 general election there were 646 constituencies in the Commons, this time there are 650. This is because there have been changes to 512 constituency boundaries across the country (although none in Scotland) and four new ones created. This is a regular exercise aimed at taking into account population movements and ensuring every seat has roughly the same number of voters.

These changes make it difficult to compare the last election result with this one. To overcome this problem, independent polling experts have created a list of how these changed constituencies would have voted if the last general election had been fought using these new boundaries. These results are called "notional" results.

The BBC - as well as other media organisations, including ITN and Sky News - use these "notional" results when assessing how the parties perform at the 2010 general election, at every level: constituency, regional and national.

In 10 seats, the "notional" results mean that a different party would have won last time than actually did.

There have been 14 by-elections since 2005 (that is, one-off elections in seats where, for example, the sitting MP has stood down or died). But, for the purposes of tallying up the results and allowing a straightforward comparison with previous general elections, all by-elections are ignored.

For example, at the 2005 election Labour held Dunfermline and West Fife but in a February 2006 by-election in the
same seat
(caused by the death of Rachel Squire), the Liberal Democrats gained it from Labour. If Labour were to win this seat back at this general election, the BBC will describe it as a Labour "win", not a Labour "hold" or "gain".

This policy has been adopted because very different circumstances often pertain to a by-election. For instance, the first by-election of the new parliament was held in Cheadle, following the death of the Lib Dem MP, Patsy Calton. The campaign proved a very bitter and personalised contest between the Lib Dems and their Conservative challengers which helped secure the biggest turnout in a by-election for five years. Third placed Labour
lost its deposit,
as its vote was squeezed by both the Lib Dems and the Conservatives.

Defections of MPs from one party to another are treated in the same way.

The Speaker of the House of Commons is an MP and has to stand for re-election in his or her constituency at every general election. Traditionally the biggest parties in the House of Commons do not stand against the Speaker,
although other parties do.

The current Speaker, John Bercow, is standing for election in
Buckingham.
The Speaker is a neutral figure in Parliament, so Mr Bercow is no longer a member of the Conservative Party as he was before his election to the role (by all the other MPs). However, for the purposes of calculating the number of seats belonging to each party - and calculating those held, gained or lost by each party - Mr Bercow is regarded as a Conservative MP and the seat is regarded as being held by the Conservatives in 2005.

What matters most is how many "seats" each party wins, and for things to change political parties need to win seats from each other. Because winning seats from each other is so important, a special language is used to show this. Seats that are won can mainly fall into two categories: "hold" or "gain".

Hold: If a party that wins a seat that it won in 2005, this is described as a "hold".

Gain: If a party wins a seat that it did NOT win at the last general election, this is called a "gain". Clearly these are really important to the opposition parties. If they are to form a new government, they need to win seats from the government and their opponents make "gains" and keep or "hold" all the seats they had last time.

There are two sets of circumstances where the BBC does NOT use "gain" or "hold" to avoid confusion.

1) Where there has been a by-election since the last general election and that by-election resulted in a different party gaining the seat compared to the general election result.

2) When the "notional" 2005 result in a constituency would have resulted in a different party winning than actually did in that constituency of the same name.

In both these circumstances the BBC uses the term "win" to describe the party that gets most votes. This applies to 14 seats.

There are four by-election seats: Dunfermline and West Fife, Crewe and Nantwich, Glasgow East and Norwich North.

There are 10 seats that, according to the "notional" results, were won by a different party than actually won the constituency of the same name in 2005: Croydon Central; Northampton South; Enfield North; Finchley & Golders Green; Sittingbourne and Sheppey; Staffordshire Moorlands; Thanet South; Wirral West; Rochdale; Solihull.

The big thing that matters in the end is which Party wins enough seats in the House of Commons to
form a government.
To do that simply one party needs to get one more seat than all the others added together. That is called an overall majority, but in the shorthand language of elections it is just called "a majority". In the 2010 elections there are 650 seats, so to get one more than everyone else put together a party must get 326 or more to get a "majority".

Of course it makes things much easier for a government if they have many more MPs than all the others put together. That number is called the "size of the majority. So, if one party were to win 326 seats, then all the other parties added together would be 324. The majority is therefore 326 minus 324: two. So the smallest majority possible is not one seat but two.

Another quick way of working this out is to take away 325 from the number of seats that winning party has got and double the result. For example:

If the winning party has 350 seats what is the majority?

350-325=25

25x2=50

So the majority is 50.

A tip for any office sweep stake on the size of the majority is never bet on an odd number. The difference between the winning party and everybody else will always be an even number.

It is, of course, possible for no party to win an overall majority. If this happens then expect to hear the term "short by". It is election short hand for the term "short of an overall majority by...". Since the lowest number needed to get a majority is 326 then to work out how many any party is "short by" just take away the number they do have from 326. For example:

A
hung parliament
happens when no single party wins a majority over all the others.

A party can stay in power
without an absolute majority
by trying to forge an alliance with a smaller party to create a coalition government, or they could reach agreements with smaller parties that they will support the government if there is a vote in parliament aimed at bringing down the government and forcing an election.

Another possibility is for the biggest party to form a minority government with no agreements with other parties and just try to form majorities in favour of each individual bill as it comes up.

If no party is prepared to go down one of these paths then parliament will be dissolved again and there will be another election.

The
last time a hung Parliament happened
in the UK was in February 1974. Labour Prime Minister Harold Wilson led a minority government, after taking just four more seats than his Conservative rival Edward Heath.

Swing is a tool which helps explain how elections are won and lost. In simple terms it is a way of measuring how the public's support of political parties changes from one election to the next.

Although it gives an apparently clear picture, the disadvantage of swing is that it can only tell you about a shift from one party to another, not the shift between three or more parties.

Swing is calculated by taking the average of one party's fall in the share of the vote and another party's gain in support.

The important point to remember when considering any individual constituency is that the swing required for the defending party to lose to the party in second place is half their percentage majority at the previous election.

For example, if Labour is defending a seat which they won over the Conservatives with a 10% majority in 2005, it would require a swing of 5% for the Conservatives to take it from them. This is based on the principle that you add 5% to the Conservative vote last time and subtract 5% from the Labour vote last time, thereby neutralising the previous 10% majority.

An exit poll is conducted by approaching voters as they leave polling stations and asking them to fill in a mock ballot paper to indicate how they have just voted.

The results of this year's exit poll, conducted at 130 polling stations up and down the country, will be broadcast at 2200 BST on 6 May.

Conducted by Ipsos MORI and gfkNOP, it has been commissioned jointly by the BBC, ITV News and Sky.

Later in the night, once sufficient results have been declared, the BBC will broadcast a prediction of the final outcome based on the results in so far. This will subsequently be updated as more results come in.

We are aware of a small number of discrepancies where a postcode search result on our election website - which is based on latest available data supplied to us by Ordnance Survey - returns a different constituency to the one given on polling cards sent to an address at the same postcode.

Normally the constituencies concerned are next to each other, and it appears these discrepancies occur when postcodes are on the border between the two constituencies.

We would advise people affected to follow the information on their polling card in terms of the constituency they are in and the polling place to be used on 6 May.

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